Q:
If matter was so dense at the time of the Big Bang, how come it
didn't form a black hole?

A:
Similar questions have bothered eminent scientists from Newton
to Einstein and beyond. The basic reason that most of the matter
in the Big Bang did not collapse immediately into a black hole,
is that the universe was expanding from a very hot state. The
expansion, plus the pressure of the intense radiation in the
very early phases of the Big Bang, had the effect of slowing
down the collapse of overly dense regions until much later. How
much later? Astrophysicists presently estimate that the first
stars and galaxies formed somewhere between a hundred million
and a billion years after the Big Bang. It is still an open
question as to whether large numbers of very massive stars
heavier than a few hundred solar masses formed at this point and
imploded to make black holes. If this happened, then massive
black holes might be the mysterious dark matter, and most of the
matter in the universe might be in the form of black
holes.